Signal transmission system for use in logging drill hole formations



Nov. 10, 1953 J, J, ARPS 2,658,725

SIGNAL TRANSMISSION SYSTEM FOR USE IN LOGGING DRILL HOLE FORMATIONS Filed Oct. 31, 1947 7 Sheets-Sheet l a o u i 1; FIG. I I

a 6/' I .50 S

6.9 FLU/D METER 1 J g; DRILL 52729 3 STRING J 74 r'\ h 1 F 7 I Z f 47 z I V 45 SUCTION SETTLI/IG HVVENTUR.

Jon Jacob Arps BY Afivs.

J. ARPS 2,658,725 ssI0N SYSTEM FOR USE ILL HOLE FORMATIONS Nov. 10, 1953 J. SIGNAL TRANSMI IN LOGGING DR 7 Sheets-Sheet 2 Filed Oct. 31, 1947 v x m 2 Nov. 10, 1953 ps 2,658,725

SIGNAL TRANSMISSION SYSTEM FOR USE IN LOGGING DRILL HOLE FORMATIONS illed Oct. 51, 1947 7 Sheets-Sheet 3 FIG. 3

FREQUENCY HEA SUR/NG NE TWORK HUD FROM E. SHALE SHAKER FIG. 4 w

PAD/O ACTIVITY RAD/O ACTIVITY CON TAN/NA T/ON MEASURE HEN T INVENTOR. Jun Jocob Arps.

J. J. ARPs SIGNAL mmsmsszou SYSTEM FOR uss Nov. 10, 1953 IN LOGGING DRILL HOLE FORMATIONS 7 Sheets-Sheet 4 Filed Oct. 31, 1947 Aida FIG 6;

MUD FROM SHALE-SHAKER INVENTOR. Jon Jocob Arps WWW Afiys.

Nov. 10, 1953 J. J; ARPS srcum. TRANSMISSION SYSTEM FORUSE IN LOGGING DRILL. HOLE F'ORMATIONS 7 Sheets-Sheet 5 Filed Oct. 31, 1947 HUD FROM SI'IALE SHAKER FIG. 7

Cu v m m r Wm M 4 0 b L w H mm c V by R m v E G M u NW A V mm m6 u NOV. 10, 1953 J, J ARPS 2,658,725

SIGNAL TRANSMISSION SYSTEM FOR USE IN LOGGING DRILL HOLE FORMATIONS Filed Oct. 31, 1947 7 Sheets-Sheet 6 msousncr MEASURING NETWORK ,4

JNVENTOR.

Jon Jocob Arps Nov. 10, 1953 J. J. ARPS ;SIGNAL TRANSMISSION SYSTEM FOR USE IN LOGGING DRILL HOLE FORMATIONS Fil d e Oct 31, 1947 7 Sheets-Sheet 7 1 l I i 7 l J i l l I l l I 250 l l l I I 626/ l L l n K s F/G. 0: .6

(arr: A

fire) FIG.

o fhs/flre Ir/IJ {j iuuuuuuuuuuufiuuuuu W uununnuununuunnnw M N INVENTOR. Jon Jacob Arps Patented Nov. 10, 1953 UNITED STATES PATENT OFF-ICE.

SIGNAL TRANSMISSION SYSTEM FOR USE IN LOGGING DRILL HOLE 'FORMA-TIONS 1 20 Claims.

The present invention relates'to methods ancl apparatus for transmitting signals f'rom one point to another through a fluid medium and more particularly to an improved method and improved apparatus for transmitting signals from a pointina bore hole to the surface of the earth.

It is the customary practice in making electrical logs' of the formations traversed by bore holes to first drill the hole, then remove the drilling equipment, and then lower into the bore hole a suitable arrangement of electrodes suspended from an electrical cable. In this sequence of operations, the necessity for interruption of the drilling operation and removal of the drilling tools from the bore hole constitutes an'undesirable feature.

Although .a few systems have been proposed for'making certain electrical measurements in bore holes while the drilling is in progress, such methods involve the extension of an insulated electrical conductor'from the surface of theearth down the bore hole, usually inside of the drill pipe, for connection to an electrode on or near the drill bit. The apparatus involved usually includes a source-of current and indicating apparatus located at the earths'surface and suitably connected to the insulated conductor and to an electrode grounded in the surface of the earth. The difficulties involved in installing and maintaining such insulated conductors in operable condition have discouraged the use of this method. In another system it hasbeen proposed to. eliminate the insulated electrical conductor and'to employ the entire drilling tool as one electrode,-and a remote surface ground as-the other electrode. Electrical measurements are then made between the two electrodes by means of measuring apparatus provided at the earths surface. It has been found, however, that such a system is not practical because the drilling tool is in electrical contact with the earth through the drilling mud for substantially the entire length of the drilling tool and for this reasonthe indications or records made by the measuring apparatus necessarily comprise the resultant electrical effect-caused by all of the strata traversed by the drilling tool. In such a system, the deeper the drill bit penetrates into'the earth the smaller willbe the proportion of the total currentflowing between the earth and the drill bit, t

and consequently the less representative the recrd will be of the geological formation in the immediate vicinity of the drill bit.

It is a general object of the present invention,

therefore, to provide an improved system and 5 method which makes it possibleto obtainlogs of 2 bore holes in the earth without the necessity for electrical connectionsextendingf'from" the surface of the earth to the position in thewell'where the nature of'theformati'on is being investigated;

Accordingltoanother object of the invention, the indication and/or recording of the nature or the geological formations traversed by a bore hole is accomplished simultaneously with the drilling operation, with "substantially the same accuracy provided by presently employed processeswhich require the'withdrawal of' the drilling tools from the bore hole.

It is a further obj ect offthe' invention top'rovide a device which is capable of electrically logging a bore hole simultaneously with the drilling thereof, thereby-not only eliminating the necessity for removing the drilling equipment and replacing it with thelogg in'g equipment, but-also providing a meanswhereby an operatorma-y tell at all times the: nature of the'formati'on through which he is drilling ins't'ead'of'havingtowait until the hole hasbeencompleted before obtaining this information.

It is still'an'other '-'object of the invention to provide a well'logging system capable'of being used during drillingoperationsand by-means of which it-is'possible to loge-characteristic of'the formation shortly after "it'has been exposed' by the'drill andb'efore there is a substantial contami'nati'on' by-penetration of drilling fluid.

Another object of the invention "is to provide means whereby information relating to the electrical and other'characterlstics of the formation surrounding the'hole being-drilled may be communicate'd'to the surface of the ground Without the'necessity for electrical conductors or other special transmitting channels-"extending from the bottom to the top of the-hole.

The-invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference-to the following specification, taken in connection with the accompanying drawings inwhich:

Fig. 1- is a longitudinal vertical section through a well being drilled with apparatus constructed in accordance with the present invention;

'Fig. 2 is a vertical cross-section through the specially constructed drill collar forming the lower portion ofthe drill stem illustrated in Fig. 1;

Fig. *3 shows diagrammatically radioactivity measuring apparatus used in the present improved system;

Fig. 4 is a schematic representation of the sequence of steps making-up the present method;

Fig. '5 is a cross-sectional view illustrating a modified drill collar which is specially constructed to provide a resistivity log and a log of spontaneous potential associated with the formations being drilled;

Fig. 6 diagrammatically illustrates radioactivity measuring apparatus used in combination with the collar shown in Fig.

Fig. 7 illustrates apparatus for automatically compensating for various fluctuations occurring in the process;

Fig. 8 illustrates a modified embodiment of the invention;

Fig. 9 is a cross-sectional view illustrating a modified embodiment of the specially constructed drill collar;

Fig. 10 schematically illustrates a pulsing circuit adapted for use in combination with the drill collar shown in Fig. 9;

Fig. 11 is a graph illustrating the variation in spontaneous potential of a typical formation adjoining a drill hole as a function of the depth of the drill hole;

Fig. 12 is a graph illustrating the variation in formation conductivity as a function of hole depth;

Figs. 13a, 13b, and 130 are graphs illustrating the nature of the voltage pulses developed at various points in the pulsing circuit illustrated in Fig. 10;

Fig. 13d is a graph illustrating the nature of the current pulses developed at the output terminals of the pulsing circuit shown in Fig. 10; and

Fig. 14 schematically illustrates radioactivity measuring apparatus adapted for use in combina 5 tion with the drill collar shown in Fig. 9.

In brief, the objects of the present invention are realized by measuring a given characteristic, such as for example the value of the resistivity, natural potential and the like, of a geologic stratum at a determined depth within a drill hole and simultaneously introducing into the stream or drilling fluid or drilling mud, at the point of measurement, a radioactive tracer in a quantity accurately representative of the measured value of the characteristic under observation. This process is continued as the drilling progresses, and as the characteristic of the formation changes, the amount of radioactive tracer introduced into the drilling fluid is correspondthe mud leaving the well and comparing it with the radioactivity of the mud entering the well, the increase in radioactivity due to the radioactive tracer can be measured. This increase is in turn a function of the change in magnitude of the characteristic being measured at the bottom of the well. Of course, if the input mud is known to be substantially free of radioactivity, then the amount of radioactivity of the output mud will be directly indicative of the magnitude of the characteristic measured at the bottom of the well. By correlating specific input and output mud radioactivity increments and by relating them to the depth of the point of measurement, the subsurface formation may readily be logged.

Referring now to the drawings and more particularly to Fig. 1 thereof, apparatus is there illustrated for drilling a well bore l in accordance with modern conventional rotary drilling practice, i. e., by the employment of a circulating hydraulic drilling fluid, such as a suspension of clay solids in water, and conventionally termed rotary mud or drilling mud to carry the drill cuttings out of the bore as the drilling operation proceeds. The upper portion of a well bore l is lined with a casing 2, generally termed the surface casing, which usually extends only a comparatively short distance into the well. At its upper end, the casing 2 is provided with a side outlet pipe which discharges into a shale-shaker 14, having the function of separating the formation cuttings carried upward by the drilling mud from the drilling mud itself. Extending into the well through the casing 2 and the well bore l is a conventional hollow drill string, designated generally by the numeral 4, which is connected at its lower end to a drill bit 5 having openings 5 provided adjacent the cutting teeth or cutting edge of the bit. A kelly joint 7 is connected to the upper end of the drill pipe and extends through a rotary table 8 mounted conventionally on the floor of a derrick. Rotation of the rotary table 3 engages the squared surfaces of the kelly joint T and thereby effects rotation of the drill string l in the usual manner to cause the bit 5 to cut through the formation. The upper end of the kelly joint 7 is connected to the usual rotary hose swivel I l and the entire drill string is suspended from a traveling block l2 which is adapted to be raised and lowered in the derrick by means of a cable 13, all in accordance with conventional practice.

Referring now more particularly to the apparatus for circulating the drilling fluid through the well bore, it is pointed out with reference to Fig. l of the drawings that this apparatus comprises the usual mud ditch 44, settling pit 45, overflow ditch 4t, pump suction pit 41, and mud pump 48 having a suction pipe 49 leading into the suction pit 47 and having a mud discharge conduit 50 which communicates with the drill pipe 5 through the swivel II. A fluid meter 5| is connected in the conduit 50 and is adapted to measure the volumetric flow of the mud fluid flowing through the conduit 50. This fluid meter may be of any conventional type adapted to measure the amount of drilling fluid flowing through the well. The depth of the well may be measured at all times by any conventional method and this is commonly done by adding together the measurements of the lengths of all sections of drill pipe in the well, and by suitably marking the kelly to determine how much of its length has descended into the well. However, the depth measurements may also be conveniently obtained by suitable mechanical or automatic measuring devices well known in the art. A suitable mechanical depth measuring device is diagrammatically represented in the drawing as a depth meter operated by means of a measuring line 6| passing over a measuring pulley 68 and leading over pulleys 62 and "53 to the traveling block 22, the measurement of the depth of the well or length of drill pipe in the well being recorded on this device in response to the downward movement of the traveling block l2 as it follows the downward movement of the drill string into the well.

That portion of the equipment shown in Fig. 1 of the drawings which is enclosed within the dashed rectangle 22a and comprises the lower end of the drill string 4 and the bit 5, is shown more in detail in Fig. 2 of the drawings. As there shown, the lower portion of the drill string, commonly called a drill collar and designated by numeral [5, is composed of one or more sections of drill pipe having increased wall'thickness, in

order to provide additional weight bearing: on'

the bit and'to decrease the whipping action which mightotherwise occur'tocause'the hole to deviate fromla straight line. A considerable portion. of the drill'c'olla'r I is covered with a wrapping or covering I6 of insulating material; Any reasonably flexible insulating material may be employed for this purpose. Alternatively, relativelyinflexible insulating: material, such, for example, as

Bakelite, may be made in the form of a cylindrical casing; which is securely fixedin place on the outside of thedrillicbllar I5; Two metallicelece trode rings I! and I8,suitably spaced one'from the other are'firmly fastened about the upper and the lower portions, respectively, of the insulating;

structure I6. Each ring may consist of a solid cylindrical structure or of thin wire conductors wound or woven togetherto give a large effective contact surface area. As pointed out below, a diflerence of potential is applied between the rings I! and I8 when the. system is in use. Accordingly, the insulator I6 is dimensioned to proje'ct" a substantial distance above and below the electrodes.

.Two insulated conductors 2B and 2! are employed to connect the electrode rings I? and I8, respectively,'to opposite terminals of the voltage supply source, whichis located above the electrode rings I I, I8. In the particular embodiment of the drillcollar under consideration, a tubular shell 28 provided with conical ends welded to one section of the drill pipe is used to by-pass the drilling fluid around a cylinder 2! mounted inside this drill pipe section and containingflthe voltage supply source. More specifically, suitable openings 25 and 26', are cut in the wall of the drill pipe above the top 23 and below the bottom 24 of the cylinder 22', respectively, so that the flow of drilling mud through the drill string in this region is through the annular space between thedrillstring and the tubular shell 28. As indicated, the space. within the cylinder 2! is occupied by a suitable voltage supply source suchas a dry cell battery 25 capable of operating for several weeks without renewal. The positive terminal of this battery is connected by means of the" insulatedconductor 20 to the upper. electrode ring I1 and-the negative terminal of the battery 29'is connected by means of the insulated conductor 2'! to'the lower electrode ring I8. An electric switch (not shown), op erated by the flowing pressure of themud' keeps the battery disconnected from the electrodes when the drill. string is being lowered or withdrawn from the hole. andwlien the mud pumps are, shut down during an interruption of the drilling operation.

Itwill' be understood that the difierence of potential between the electrode rings I! and I8 causes a. current to flow along'the path indicated by dashed lines 30. A portion of this current flows through .the' drilling fluid and the earth's formation in the immediate neighborhood of. the drill bit. More in detail, thecomplete electrical circuit followed by the flow of current extends from the positive terminal of the battery 29 by way ofthe insulated conductor 20, the electrode ring I!., the mud section3I adjacent the electrode ring I!, the adjoining portions of the earths formation, said'forrn'ations being located predominantlywithin the regions 32,the mud; section 33., the electrode ring 7 I8 adjacentthereto, and the insulated conductor 2!. back to the negative terminal of the battery 29.

The geonitryof. the circulating. systemis soiam ranged that the resistance of. the mud'sections 3 I and 33 is small as comparedwiththe-resistance.

of the formationthr'ougli' which the currentis flowing. Consequently, the magnitude of cur rent flow in the above. traced circuit depends to a very large extent upon the conductivity of'the formation in the region 32. .As the drilling operation progresses, the drill bit encounters in its downward travel formations of varying conductivity and, accordingly, the magnitude of current flow in they described circuit .variesv substantially proportionately to the conductivity. of

the formation encountered. Such an electrode.

arrangement is well known in the art and. is commonly used in electrical resistivity well log- In accordance with the present. invention, the upper electrode I! is preferably made ofamaterial which is'both electrically conductive and radioactive. When current passes from the,elec;- trode into the mud stream, it does so because the mud acts as an electrolyte and by a suite able selection of the electrode materials and of the voltage of the battery 29, the passage of the current into the electrolyte is accompanied by a migration of some of the material of the electrode into the mud stream. It is well known that when a suitable difierence of potential is applied between two electrodes such asrthe electrodes I! and I8 immersed in a conducting solution consisting of the drilling fluid therebetween, the magnitude of the current flowing through the drilling fluid depends not only upon the applied voltage, but also upon the extent to. While an opposing electromotive force is built up at the surfaceof the electrodes. The production of this counter electromotive force, commonly knownas polarization, .is caused by the electrolytic deposition of material on one or both of the electrodes. It will be apparent that in the arrangement shown inFig. 2 of the drawings, the electrodes may be depolarized in a conventional manner by removal of the cause of polarization. Thus a precipitate or a gas may dissolve or diffuseaway the polarization products, or they may be removed or neutralized by chemical action or prevented .fromformin'g by usinga suitable polarityreversing .switch in the circuit. In case metal electrodes I! and !8 are used, it is not necessary that the metal from which electrode If! is'constructed and which is sent into solution by the electric current itself beradioactive; e. g., the electrode may be made out of copper or lead in which radioactive phosphorus has been dissolved. Whenthis copper or lead is sent into solution under. the influence of the electric current, theradioactive phosphorus particles are re-. leased .and carried upward by the mudstream. This phosphorus ,will oxidize and combine into phosphates while still retaining its radioactive characteristics. Another advantage of using such radioactive metalloid impurities in the electrode metal is that. these impurities do not form positively charged ions and are therefore not subject to redeposition on the other electrode. Thusit. is possible, through the use of a suitable alternating switch in the circuit, to alternately make each of the electrodes positive, thereby avoiding polarization effects.

If the. drilling fluid separatin the electrodev rings I!, I8 were stationary, the radioactive ions entering the solution. from the electrode i=1 wouldbe carried slowly downward by the existing electrostatic field and subsequently be deposited on the electrode [8. Under ordinary drilling conditions, however, the mud surrounding the drill string is usually in a state of rapid motion toward the top of the drill hole, 1. e., in a direction opposite the very slow downward travel of the radioactive ions. Consequently, practically all of the radioactive ions released into the mud are immediately removed from the zone in which the electric field is present and remain in the mud as it is pumped to the earths surface. As a result, they can be subsequently detected at any convenient location at the earths surface. More over, the amount of radioactive material released into the mud is proportional to the magnitude of current flow in the above described cir cuit, and consequently is at any instant representative of the conductivity of the formation. Thus the mud in the immediate neighborhood of the electrode I1 is rendered radioactive to an extent directly representative of variations in the nature of the strata being drilled.

From the above explanation it will be evident that as the drilling operation proceeds, varying portions of a suitable radioactive substance are dispersed in successive increments of the drilling fluid column rising in the well, and although dispersed therein in exceedingly diluted proportions, the dispersed radioactive ions will hold their respective positions in the respective increments of the drilling fluid column into which they are dispersed. In accordance with this invention, a novel and useful method and apparatus has thus been devised whereby successive increments of the drilling fluid returning to the top of the well may be analyzed to determine the radioactivity in such successive increments. Furthermore, the movement of each increment of the drilling fluid may be traced in its passage through the well, and by suitable correlation with the depth of the stratum, measured in synchronism with the rise of the increments of the drilling fluid from that stratum, it becomes possible to determine the arrival at the top of the well of each increment of the drilling fluid with its radioactive content and to then measure the radioactivity of that increment and identify the resulting measurement with the stratum associated therewith.

Generally stated, and in accordance with the illustrative embodiment of this invention, a radioactive tracer is utilized to represent the resistivity of the formation drilled and is gradually dispersed in the neighborhood of said formation in successive increments of the drilling fluid column rising in the well. The location of the formation may be ascertained by determining the rate at which the circulation of the mud occurs, as by considering the rate of pumpage of the mud, the rate of penetration of the bit and by measurement of the depth of the formation at which the mud acquired the observed radioactivity. Also in accordance with this invention, use is made of the fact that hydraulic fluid flow through the well is in the form of a closely restricted stream, the incoming fluid being confined within the bore of a drill string, while the outgoing drilling fluid is confined within the annular space between the wall of the well bore and the outside of the drill string. While the drill string may be rotating at fairly high speed in contact with the drilling fluid moving upward on the outside of the drill string, very little mixing of lineally spaced increments of the fluid stream is known to occur. Therefore, the only changes produced in the radioactivity of any increment of the drilling fluid, are developed in the immediate vicinity of the electrode l1 and are produced by introduction into the fluid of variable quantities of the radioactive tracer substance. Consequently, by tracing said radioactive substance in its rise from that stratum to the top of the well, and by then analyzing its radioactivity, the electrical resistivity of the stratum can be determined, and its location in the well can be properly logged.

In order to measure the amount of radioactivity present in the drilling fluid a the fluid enters and leaves the well, suitable radioactivity detectors are provided which are respectively contained within vessels 10 and H and are respectively equipped with indicators 12 and 13 for providing indications of the amounts of radioactivity in the drilling fluid at the time when the measurements are performed. More specifically, the container 10 is connected in the suction pipe 49 of the drilling fluid circulating system and the radioactivity detector contained therein controls the indicator 12 to provide an indication of the radioactivity in the drilling fluid as the fluid is pumped into the well. The container II is connected in the outlet pipe 3 of the circulating system downstream from the shale shaker M and the radioactivity detector contained therein governs the indicator 13 to provide an indication of the radioactivity in the drilling fluid as the fluid is pumped out of the well. The detectors housed by the containers 10 and H are identical and accordingly only that contained within the container H has been illustrated.

This detector, generally designated by the reference character 90 in Fig. 3 of the drawings, may be of any type well known in the art which is adapted to respond to beta or gamma radiations, derived from minute amounts of radioactive substance present in the drilling fluid. It consists of an envelope 9| of convenient shape and material, such, for example, as glass, and provided with one or more aperture 92 covered with a material such as mica or aluminum, of suflicient mechanical strength to exclude the drilling fluid and at the same time allow beta particles emitted from the radioactive substance within the drilling fluid to easily pass into the detector chamber. In case gamma radiations are to be detected, the covered aperture can be dispensed with since gamma radiations pass through relatively thick metallic walls. Two or more electrodes 93 and 94, which may have various geometrical shapes and relative positions, such as, for instance, rectangular grids whose planes are parallel, or coaxial concentric cylinders, are introduced into the envelope 9| and the Whole is rendered gas tight by any suitable means. The interior is evacuated and a suitable quantity of gas, such as, for example, neon, is introduced until a suitable pressure corresponding to from five to ten cm. of mercury is obtained. The electrodes 93 and 94 are connected by leads 95 to an electrical circuit consisting of a battery 96 connected in series with a resistor 91. This resistor is connected to the input terminals of a frequency measuring network 98, the output terminals of which are connected to the input terminals of the indicating meter 99. The frequency measuring network may be of the type described as A Counting Rate Meter for Radioactivity Measurements in The General Radio Experimenter, vol. XXII, Nos. 2 and 3, July-August 1947. It functions to convert a succession of received current impulses into a direct voltage having a magnitude representative of the number of current impulsesheeeiyedner un .,e.fa-time- --'-I'he;-. h li %t9r 99- may; bea ta .m liv ltmeter- I t-th -.op at. on f the detector 19hand; a s ela ed equip n the vo ta o th hatter it s ad usted :to ra su tah eryalue and.-,h ta. e amma) 1- particles enterinecthetenc esed ionizat on chamber cithe dete to are rec rded ta tell w A; b ta .(or; gamma pa tiele. emitte rom the rad active -v 1 ee :wit i :t dri l n n ehamb reh z ro hee or .at ee closed ee ;U derthe nfluence .of :the lec ic fie d ex stin betwee the electrodes 93 d 94 o fthe ideteetorliehieetio takes,- laceh eoll-isio pt asmo eehle an mean. .t impale is ehe ted hi h-traverse th -e eu t-eomprism th reel ange fl'l thggurrent south 18 an 5th 7 .eetrode 83. ahdfik Of the detect Th seu h imnulseeau e aee res hhd nevoltaee her u to be tde e oped heires th resietor leqhehe w t wh an hlnuls etr vers the re istorfl is-hf eterm ned by; the ra e a h ehthe rad. ia tir pa tic es; enter t e ion za ion c amber of t e ,d teeter.- This req e c determ ne the, ma niud oi-theyolt se developed tth Qu ht t rmihelsof th n wer i hflza dhe ee; th ihd eatihh p ov ded.hy heme enS Qomearison be ween; he-rad oactiv y; onten of .1 th inpu and ou-thutrm x t thewel is had a drill-hole. C orreotio fo-r this fected by utilizin ;filleu ll ilashigllsnfi h f i meter-mend t de th hdie or 6 the manne el k W in vh 4 t edh eeripti o a. met d e eee a y t e rre eh ooeeaeienedth l th idee M ..ed thh lee a oi a plyin -the eeorree i hetdmeaeh eme 9 nnu ande tnutheu u e a -b tou ed to tam n Uh te tate l-h teht 51 223922 3 i s -to hhh aliaywa d-en 'E bruern zr 1%?- 1 eahev eahla ietiehi it wil he ahhareh tha th e ectrode. ma cont an hstah cap bl f srioht neoushh lear v nte at hh and havh aja rde ired hal pl feh ried- 1 119 ,a .h tehe rma he lohahve such as adihm zh i have a eehsiderehlye herter ha l .o th order of magh tude o thetimeintervalequ red t t tracer tor-com let v the path. from t e bot om o he drill: hole to; heheart f oeur eee- 'ilhemosteuitahb lifetim o r dio ive-tr ce an-robe. determined from a c nsiderati n of Fig. 4 ft w t-drawin s, I whiehisohemat cally represents. one omplete-cycle of voneratiens: nvolve n; th pro s :Inthis cye1 ,..th v1101.1, t-An aresentsrthetinstant atwhich a determined, portion of radioactive material entersthe mud stream, and; the point B; represents a; subsequent instant at whichisaid p rtion-11s measured at the ear h surface. .The,.b1ock T1.represents the timeidelay caused-by the mud traveling. upwardowithing the annular space bounded by thezdrillwpipeon the one side of the wall of theborehole or casing on the other, and the block I03 represents the time t va I durin th fra here hen r lha' h li th slu .P tahdth ere ur sthrough the d lhihe r m thesuryfee t the; sub, rfaee- Under cond tions no ma l foun i dr hihae e a e l h. h--. 99 o e 01: exame mth delarir re r sented by t ehlocko it! t mamequal min tes; d a large. 11-l h-D =iS employed the delay T2 repreehnt di hr the: loc .l 03 may {time f ee-fi ldi -as hlh heee u ou Inuc a sys em i is; able t in ec in o t e y h et eemt mete c apable of carrying. the information, to the 'hr e by hlaiht ihih its ad oacti f a interval greater than the period T1, but which; tor neried t eme hi I but sm t a T decay -zr iea ti int -t ahee e amount- Ln the example given above where -T1;equals 2 5 m1 es and T; equals four hours, therefore, 7 it i esirableto use a radioactive substance havhaara h e yeheh h the peri T H' SZ: mea ur l ra i vi Still ex st an that after the period Tz expires the radioactivity is jnon z-rn surable. gomeof the radioactive elemehtee es ih th q remen s a a h lows:

( 1- having aha-1f life of 35-min e @2 h -lg? lf life of 40 minutes $11111 havinga half life of 44 minutes I havinga half life of 25 minutes ing mud, and correlate the radioactivity measurernent with the reading of the fluid meter in the manner described above inorder to log the measured characteristic of the ubsurface strata under observation.

Referring nowfmore particularly to Figs. 5 and 6;.of the drawings, the modified system there i1- lustrated is adapted'simultaneously to measure and record two different characteristics of a 51115- su'rface stratum, such, for example, as the resistivity and the natural potential; To this end and as-best shownin-Fig.5 or the drawings, the drill bit I05 is electrically insulated from the drill collar I I5 in order that a potentialdifiefeme may be developed therebetwee'n. 'jThis is accomplished by; inter-posing a sleeve ms of insulating material between the bit and 01131; The drill collar H5 is -v maintained at the potential fof the earth through its connection with the drill string which is in continuous-contact with the surface casing and the upper part of-thedrill hole and also with the. mud surrounding the drillstring throughout thelength of the hole. This collar is provided with Van elongated insulating cylinder and two pairsofelectrode rings: I I1, I I8 and I40, III which surround the cylinder. :Theelectrode rings I I I 1, H8 areelectricallyconnected to a battery I 29 positioned withinasuitable chamber formed within the drillpipe in such manner that-flower the drilling-fiuidthrough the ldrillpipe is not interfered with, and the differenceofwpotential-thusldeveloped hbetween the..electrode. rings. II! and II8 cause a current: torflow through the adjoining formation along the paths indicated by. the dashed lines: I20. 'r-lt will-be understood that thevalue of the current flowing from the electrode I IT; to the electrode, I I 8.;depends to a very large extent upon the conductivity of the formation. :The electrode II'I contains a suitable radioactive substance which under the influenceiof the current becomes gradually: dissolved the, :drilling :fluid; the rate of migration of radioactive ions from the electrode into the fluid depending at any instant upon the magnitude of current flow and hence upon the conductivity of the adjoining formation,

It will thus be apparent that the arrangement comprising the electrodes II! and I I8 and battery I29 is similar to the arrangement within the dotted rectangle 22a and performs the same function, i. e., that of introducing into the drilling fluid a radioactive tracer substance in quantities representative of the resistivity of formation in the neighborhood of electrodes I7 and I8. The Fig. 5 arrangement is, however, characterized by one distinguishing feature, viz, the disintegration of the radioactive substance in the electrode II"! is accompanied by the emission of two different radiations such as beta and gamma rays. Such a substance may contain, for example, Mg. (or Ca, or K). Consequently, the amount of tracer in the drilling fluid can be determined not alone by a beta ray detector but also by means of a gamma ray detector. Hence, the intensity of the gamma radiation is representative of the quantity of 0a, or Mg, or K in the drilling fluid and may be used as an indication of the conductivity of the formation drilled.

The closely spaced electrode rings I40 and MI surrounding the insulating cylinder H6 at a suitable distance above the electrode rings Ill and IIB, are part of a second measuring system adapted to measure the spontaneous potential generated at the interface between the formation and the drilling fluid. It will be understood that because of electroflltration and electrochemical effects, a difference of potential occurs spontaneously between the insulated drill bit I95 and the grounded drill collar H5. The

drill bit and the drill collar are connected by I means of circuit leads I32 and I33, respectively, to an amplifier I34 positioned within the drill collar in a manner such that the flow of the drilling fluid through the drill string is not interfered with. The input voltage to the amplifier I34 is in the millivolt range and represents the spontaneous potential generated at the interface between the neighboring formations. This voltage is amplified by the amplifier I35 and applied across the electrodes I49 and MI. The electrode I49 contains a radioactive substance having properties different from the properties of the substance contained within the electrode Ill. Specifically, the substance in the electrode I49 contains an element such as *Cu which emits beta radiations only, while the substance in the electrode II'I emits both beta and gamma radiations. It thus becomes apparent that under the influence of the varying current derived from the amplifier I34 a varying amount of radioactive copper is continuously dissolved in the drilling fluid, the amount being at any instant representative of the spontaneous potential difference between the drilling bit I95 and the grounded drill collar II5.

As in the system shown in Fig. 1 of the drawings, the electrostatic field produced between the electrodes I40 and MI tends to move the ions of radioactive copper downwardly from the electrode I49 to the electrode I4I. Similarly, the electrostatic field produced between the electrodes III and H8 tends to move the ions of radioactive magnesium downwardly from the electrode III toward the electrode II8. However, because of the much greater upward motion of the mud stream, the radioactive ions of copper and magnesium remain in the drilling fluid and are carried to the earths surface. A mixture of two different radioactive tracers, such as radioactive magnesium and radioactive copper, is thus obtained in the mud stream, and subsequently at the earths surface. The radioactive magnesium is discharged into the mud stream in quantities representing the conductivity of formations encountered, and the radioactive copper is discharged into the mud stream in quantities representing the magnitude of the spontaneous potential.

In order separately to identify the above two tracers and to obtain separate measurements of resistivity and spontaneous potential, there is provided at the outlet pipe 3 a detector unit comprising two detectors I99a and I992). These detectors are shown in Fig. 6 of the drawings as being disposed within a container I'II connected in the outlet pipe 3 of the mud circulating system shown in Fig. 1. The detector I901) is provided with steel walls of such thickness as to stop all beta radiations and to allow only gamma radiations to pass through its walls into the ionization chamber for detection. The detector [9911, on the other hand, has a substantial portion of its walls made of a thin aluminum sheet, in order to allow the beta rays to pass into the gaseous medium of the ionization chamber for detection. A circuit serially including the electrodes of the detector I90a, a battery IBM and a resistor I9la is used to translate the detection of radioactive ions by the detector ISUa into voltage impulses which are impressed across a frequency measuring network I98a having the function of delivering to the millivolt meter I991), a voltage proportional in magnitude to the frequency of the detected voltage pulses. Similarly the frequency measuring network I981) cooperates with the circuit elements I98b and I912) and the detector I992) to deliver a direct voltage to the meter I99b having a magnitude which is proportional to the rate of radioactive ion detection by the detector I901). Thus the meter I991) functions to indicate the intensity of gamma ray detection by the detector I991), i. e., the amount of radioactive magnesium in the mud stream, and the meter I99a functions to indicate the combined intensity of gamma and beta ray detection by the meter I99a, i. e., the amount of radioactive copper in the mud stream. If at any given instant the indication provided by the meter I991) is N1, it can be assumed that N1 represents the conductivity of the subsurface formation adjoining the point at which the tracer was dissolved. On the other hand, the indication provided at any instant by the meter I99a, which may be assumed to be N2, represents the gamma and beta radiation, 1. e., it corresponds to the combined contribution of both the radioactive magnesium and the radioactive copper of which one represents conductivity and the other represents the variation in spontaneous potential. Consequently,

where K1 and K2 are appropriate constants and N3 represents the variation in the spontaneous potential. From the indications provided by the meters I99a and I991), the values N1 and N3 may thus be obtained, the first representing subsurface conductivity and the second representing variations in the spontaneous potential associated with the formation being drilled.

ramme In certain cases: it is: possible:v to: use: an 9186+,

trode I! as shown in Fig; Z of:thedrawingsimadel of a material which. is notradioactive-but which;

is caused-to become radioactive when irradiated with neutrons. If this system. is, used,\suitableopenings, may be provided inthewall of thedrill collar [5 in radial alignment with the electrode- I *i to receive the-neutron source material. It is well known that a numberofelements desiderable range:

as T in" each instance:

(T=2;7 days) Assume, for. instance, that the electrode IT contains manganese and, that the neutron source, consists of standard, radium-beryllium mixture surrounded by a layer of paraffin to slow down the neutrons, the mixture being, produced, as a result of the bombardmentofthe beryllium targetv by alpha raysfrom. radium. The slow neutrons thus produced enter into the electrode IT and'render the manganese atoms radioactive in accordance with the reaction ofElquation 4'above. The radioactive. isotope of manganese produced as a. result'of this reactionemits beta radiation. andhasa half life time of 2.5 hours; It will'thus. be apparent that in accordance with" the process describedabove; the amount of radioactive mane g-anese dissolved in the mud will" beat anytime proportionaltothe conductivityofthe'formations, traversed'by the drillbit: It serves as a tracer.

representative of the subsurface conductivity, and.

is carried by the mud" stream to the' top ofthe' drill hole'to produce the above-describedresponse! of the radioactivity detector H; with the result" that the meter" providesan indication of'the.

conductivity of the formation; The'electrode l1 may b'econstructedof other materials; containing'; for example; sodium, magnesium;- potassium; nickel, etc:, in'which cases and in accordance'with the reactionsof Equations 1,2, 3; 5, etc;, shown above; the-radioactive tracer emitted-willibe the' beta rayemitting isotope of the corresponding,

element, having'half'lives of 12 hours,- -minutes; 14 hours, 2;? hours, etc., respectively.-

It should be pointed out that in the abovedescribed system, the concentrationof thGIfidiOr active tracercontainedin the mudstream may;

vary forreasons other than" changes in formation-resistivity, such as fiuctuationsin thevelocity of the drillingmud due to theemployment of,

different pumps or varying pump speeds; and a' decreasein the concentration--01? the radioactive material employed as a tracervoccasioned by -its' g-radualdecay with time. Variousmethods may readily be-employed to compensate forthesevari- 1'14 ations; and: one of such methcds :is instrumenttedt through use of the apparatus ShOWHlziHlF-JQLTOfQ the drawings, which should. be used in conjunction with the drill collar. and bitassembly showna in Fig, 2.7 This apparatus is in the. formcofra.

Wheatstonebridge which comprises four arms: as follows: an ionization .chamberAllll; anionization: chamber 49.1, aresistor 402'; andaresistor 4031 Phebridge circuit is energized by aLbattery 40,42 connectedbetweentwocopposed1terminalsrofth69 circuit; and the output voltage of the. circuit as: developedcacross; the other. two terminals-thereof; is: delivered. to" a. direct current amplifier: NNS:- This amplifieiadeliverszits output current through;

i an electrodeiassembly 495' to'. thegalvanometer;

coil ofyarecording elementembodied in axstripp recorder 49]; The: two :ionization chamberslllflri and: 491 are of: atconventionalwtype preferablya' havinga capacity of I 1 liter and fillediwith argon: at lODJratmospheres; The resistorsllflzi anda403i have values of 101 and 12X 10 3 ohms, respective-15121 The amplifier 495is aconventional direct lcure rent amplifier capable of a large.- direct zcurrentt output, and the electrodes. 406; consist of: two; groups, each comprising a series. of spaced metal lic electrodeplates; The electrode: plates: beer longingto these. two :grou-psare alternatelyrin terconnected as shown. inthe figure andithel anode 'group of plates contains azradioactive ma? terial which is identical-to.:the:one: used" inwth'e electrode ring I1, shown in-Fig. 2 at thelower end;of the drillstring. Itwill belseen that the resistor 453 has twicethe-w ohmic value of: thee resistor 502 and, consequently, the-Wheatstone j bridge isbalanced only when the resistance-oil the ionization chamber- MiG-is twice theresist ance of the ionization-chamber ADI, i; e., when the radioactivity. of the mud in the immediate neighborhood of the'ionization'chamber 400 is one-half the radioactivity of the mud in the' immediate neighborhood of the ionization chamberAOl.

In the operation of the detecting apparatus just described, the electrodes 405; when supplied by a current from the amplifier 495 placethe radioactive tracer material in solution" in the mud at arate determined by the 'magnitude of' the output current of the 'amplifierflflfia- When the radioactivity in the neighborhoodof the ionization chamber 4M is less than twice the" radioactivity inthe neighborhood-of the ionization chamber-605*; the Wheatstone'bridge'isunbalanced" and a voltage representative of the degree of'this unbalance is supplied tothe am-r plifier M35. Since the output terminals ofithe amplifier are connected to deliver theamplifiers output current to the electrodes 496, the currentl passing between the electrodes-50E in-rthe .dri1l ingrfiuid .is proportional vtothe unbalance in the; Wheatstone'bridge. The. radioactive. material of, which.the electrodese lfldare, made is-dissolveds in the drilling. fluid: in accordance with the varia-- tion ofv the-currenttraversing theeelectrodessin the exact. manner. previously explained: This: process continues until the radioactivity; of ther mudrin: the neighborhood of the'ionization:cham:-!-- her 461 is- -raisedto 1a bridgeibalance value of al most twice-the value of the radioactivityvir'r the neighborhoodof theionizationchamber 400;" As this balance is -ap-proach'ed, the voltage input to the amplifier 405'becomes exceedingly small,'i. er, almost zero, and the output ct-the amplifier-likewise becomes exceedingly' small. An approach'tv a -condition of complete balance" can" be realized if the amplification provided by the amplifier 495 is sufliciently high.

Assume now that the radioactivity of the mud stream flowing out of the drill hole towards the ionization chamber lite increases from a value at which a balanced condition of the system prevails. In response to this radioactivity increase, the output voltage of the bridge circuit immediately increases, thus causing more radioactive material to be dissolved into the mud stream by the electrodes 338 until the system balance is again established. On the other hand, if the radioactivity oi the mud coming out of the casing towards the ionization chamber 1% decreases, then the output voltage of the bridge circuit is correspondingly decreased, thus causing less radioactive material to be dissolved into the mud stream by the electrodes M6 until a condition of system balance is again established. It will thus be understood that the radioactivity in the neighborhood of the ionization chamber illl is always maintained about twice the radioactivity in the neighborhood of ionization chamber M30, and that the unbalance or output voltage of the Wheatstone bridge circuit is a measure of the radioactivity which is added to the mud stream to maintain such a relationship. It will also be apparent that the current passing through the elec trodes 4B6 varies with the radioactivity content of the drilling mud in accordance with a relationship that is similar to the one between the current passing through the electrode ring ii and the corresponding amount of radioactive tracer dissolved into the mud. The arrangement comprising the electrode ring I! can be visualized as a transducer converting the different current intensities into corresponding quantities of radioactivity. The arrangement comprising the electrodes 436 is an inverse transducer, which reconverts the radioactivity that has migrated to the earths surface back into an electrical current which flows from the output terminals of the amplifier 695. The magnitude of this current is recorded by the strip recorder 40? and provides a reliable index of the formation conductivity which is obviously made independent of fluctuations of the mud velocity and of the natural decay of the radioactive tracer material in electrode I1.

In Fig. 8 of the drawings there is illustrated a modified embodiment of the present invention which does not involve any resistivity measureents and subsequent use of radioactive tracers serving as a resistivity index, but is based upon direct detection of various substances present in the formation. The process consists in direct irradiation by a stream of neutrons of the for mation drilled, whereby a number of substances present in the formation become radioactive, subsequently utilizing the mud stream to carry these substances to the top of the drill hole, and in measuring radiations derived from said substances. In general, the arrangement for carrying out the process is illustrated in Fig. l in which, however, the lower portion of the drilling equipment included within the dotted rectangle 22a is replaced by the drill bit and collar arrangement diagrammatically represented in Fig. 8. As there shown, the lower end 286 of the drill string is connected to a drill bit 265 of conventional design. Within the drill bit 205 two openings 286 and 2m are provided which contain a source of neutrons such, for example, as a radium-beryllium mixture. As a result of neutron bombardment of the adjoining formation nuclear transmutations take place, some 16 of which are as follows, the half lives being designated as T in each instance:

As a result of these transmutations, various radioactive isotopes such as Cl, Ca, Fe, A1 are produced under the effect of neutron bombardment. These radioactive isotopes will normally be contained in the comparatively small cylinder of cuttings drilled out by the bit, and Will become dilutedly dispersed by the action of the drill bit in the upwardly rising column of drilling fluid, and may, by suitable method of analysis, be detected in the drilling fluid upon its return to the top of the drill hole. During the drilling operation, the successive portions of the material drilled from the strata, and the radioactive isotopes contained therein, are dispersed in successive increments of the drilling fluid column rising in the well, and although dispersed therein in exceedingly diluted proportions, the dispersed contents will retain their respective positions in the respective increinents of the drilling fluid into which they are dispersed. In accordance, therefore, with this invention, a novel and useful method and apparatus has been devised whereby successive increments of the drilling fluid returning to the top of the well may be analyzed to determine the presence of radioactive isotopes in such successive increments. Furthermore, the movement of each increment of the drilling fluid may be traced in its passage through the well, and by suitable correlation with the depth of the stratum, measured in synchronism with the rise of the increments of the drilling fluid from that stratum, it becomes possible to analyze the in crement and correlate the resulting analysis with the stratum responsible therefor.

In certain of the above-described embodiments of the invention, the radio active tracer is continuously dissolved in the circulating fluid in amounts representative of the subsurface iormation characteristic under observation. Since the process is continuous, it may eventually lead to a concentration of radioactive substance in the drilling fluid which exceeds the permissible limit for performing a satisfactory measurement. It is obviously desirable to maintain the radioactive concentration at a relatively low value. The concentration of the radioactive substance in the drilling fluid is proportional to the half life of the substance and to its rate of dissolution in the drilling fluid, and is inversely proportional to the volume of drilling fluid used in the circulatory system. Therefore, concentration of the radioactive substance in the drilling fluid is most likel to occur when radioactive tracers of relatively long half life are used in drilling wells of small diameter having correspondingly small mud pits associated therewith.

In order to utilize radioactive tracers of long life and at the same time keep the concentration of radioactivity relatively low, i. e., within the permissible limit, a modified embodiment of the present improved system may be employed in which the radioactive tracer substance is intermittently released in small quantities at determined instants, i. e., in short impulses separated by suitable time intervals. To this end, the apparatus illustrated in Figs. 9 and 10 of the drawings may: be employed intermittently to-releasethe radioactive tracer intothe drilling fluid traversing the fluid circulating system. In brief, and as best shown in Fig. 9 of the drawings, this apparatus comprises a specially constructed drill tool attached to the lower end or the drill string and consisting of a drill bit 25l' and drill collar 252 electrically insulated from each other by means of an appropriate rubber sleeve 253 interposed ltherebetween. The drill collar is maintained atearth potential through its connection with the drill string which is in continuous contact with the surface casing and the upper part of the bore-hole and also with the mud surrounding the drill string throughout thelength of the drill hole. It carries an elongated insulating sleeve 254 and a pair of spaced electrode rings 255" and 256 which clampingly embrace the sleeve. The electrode rings are electrically connected to the output terminals of a current pulse generator 25-! which is'positioned within a suitable opening'in the drill pipe in such manner as not to interfere with the circulation of the drilling fluid in the circulating system, and is provided with input terminals 258 and 259 connected respectively to the drill bit 25l and to the grounded drill collar 252.

The impulse generator 251 may be of the character illustrated in Fig. 10- of the drawings. In brief, this generator comprises a relaxation oscillator stage comprising a gas filled electron discharge tube 214 and a pulse shaping stage comprising an electron'discharge-tube 282. More specifically, the tube 214 is" of the well known tetrode type provided with input electrodes, i. e., a control grid 213 and cathode 215, coupled to the generator input terminals through a cathode resistor 215, a grid resistor 2.12 and a grid biasing C-battery 2'. A condenser 218 is connected between'the cathode and control grid of the tube 214 Ito provide a path for the current pulses traversing the input circuit of this tube during operation of the generator. The output circuit of-the oscilla tor stage conventionally comprises a condenser 28!! con: nected in series withthe cathode resistor 215 across the cathode and anode of the tube 214; Voltage pulses developed across the cathode resistor 215 are impressed between the input'electrodes 283 and 285 of thetube 282 through a coupling network which comprises a grid resistor 286, a cathode resistor 284, a coupling condenser 281' and resistor 2 88 and a grid bias ing C-battery 289. Anode current is supplied'to the tube 282 from a battery 298, and theoultput voltage pulses developed across the resistor are directly applied to the output terminals 280 and 26!. The battery 29!! also supplies charging current to the condenser 280 through a re= sistor 29l having the function of determining the rate of charging of the identified condenser.

In considering the operation of the impulse generator 25?, it will be understood that the spontaneous potential developed between the electrode ring 258 and the drill bit. 25! is impressed between the input terminals 258 and 259, and that the voltage pulses developedacross the output terminals 268 and 26! are impressed between the electrode rings 255 and 256. More specifically, the variations in the spontaneous potential andconductivity of the. formation adjacent the drill tool may be graphically illustrated in the manner shown in Fig. 11, curve A, and in Fig. 12' of the drawings, respectively. In both figures, theaabscissa' represent depth: in; feet; In Fig. 11, curve A, theordinatev line-is scaled in terms of: spontaneous potential (in negative volts) occurring at various depths in the drill hole, and in Fig. 12 the ordinate line is sealed in terms of formation conductivity at various depths in the drill hole.

When a voltage varying in accordance with curve A of Fig. 11 is impressed across the input terminals 258 and 259 of the impulse generator, a voltage of identical pattern but shifted in absolutevalue by an amount equal to the voltage of the biasing 'battery'21 l is negatively applied to the control grid 278 of the oscillator tube 214. More specifically, the battery 21! has the effect of shifting the input voltage-pattern to the tube 214 in a negative sense in the manner shown by curve B in Fig. 11. It will be noted" that this voltage is always negative. The tube 214 and its associated input and output circuits are connected to function as a conventional relaxation oscillator, such that the current impulses developed in the oscillator output circuit vary both in magnitude and in frequency in accordance with the magnitude of the voltage negatively applied to the grid 213 of the oscillator tube. The anode current pulses thus caused to traverse the space current path of the tube 214 result in the production of a voltage V2 across the condenser 28!) having the pulse pattern illustrated in Fig. 13(a) and in the production of a voltage V3 across the resistor 215 having the pulse pattern illustrated in Fig. 13(1)). It will be noted that'both of these voltages vary in magnitude in accordance with varia tions in the input voltage as represented by curve B of Fig. 11, and that the frequency of the pulse components making up these voltages similarly varies directly in accordance with the magnitude of the generator input voltage; The oscillator output voltage as'developed'across the oathode resistor 215. is'impressed across the input circuit resistor 2880f the tube 282Jthrough the coupling condenser28'l in the correct sense to drive the grid 285 of the tube 282 positive. This voltage is. opposed by the'fixed'voltage of the biasing battery 289. The resultant voltage is negatively applied to the grid 285 of the tube 282 through the grid resistor-286' and due to the pulsed characteristic thereof. causes: current pulses to be developed. in the output circuit of the tube 282. More specifically, the biasing battery 289 has a voltage suflicient to bias the tube 282 beyond cutoffwhen the oscillator section of the generator is quiescent. Each pulse of the voltage V3, on the other hand, has a magnitude sufiicient to drive thegrid of the tube 282 positive even though opposed by the voltage of the battery 289. However, the. resistor 286 prevents the tube 282 from drawing grid current and hence prevents the grid 285 from being driven positive relative to the cathode 283.- As a result, the voltage V4 applied between the input electrodes of the tube 282 varies back and forth between a zero value and the negative cutoff value determined by the voltage of the battery 289 in the manner illustrated in Fig. 13(0). The tube 282-is thus alternately biased to cutoff and rendered fully conductive in accordance with the cyclic variations of the voltcy in accordance with variations in the input voltage as represented by the curve B aredeveloped across the resistor 286. It will thus be seen that the network interconnecting the oscillator output resistor 215 with the input electrodes of the tube 282 functions to prevent the voltage impulses developed across the resistor 28 l from varying in magnitude in accordance with the variations in magnitude of the voltage pulses appearing across the resistor 215. This is of importance since it is necessary to apply voltage impulses of a substantially constant amplitude to the ring electrodes 255 and 256 if the current flowing between these electrodes is to be made to vary in accordance with the conductivity of the current path extending therebetween.

As shown in Fig. 9 of the drawings, the output current of the pulse generator traverses a circuit comprising the electrode ring 255, the drilling fluid section 282 adjacent the electrode ring 255, the adjoining portion of the earths formation which is located predominantly within the region 263, the drilling fluid section 284 and the electrode ring 256. As will be apparent, the geometry of this circuit is such that the resistance of the drilling fluid sections 262 and 264 is small compared to the resistance of the formation through which the current is flowing. Consequently, the resistance of the circuit across the output terminals 286 and 26! of the impulse generator 251 and hence the magnitude of the current traversing this circuit depend substantially entirely upon the conductivity of the formation adjacent the drill tool. Thus the impulse generator 251 receives at its input terminals 258 and 259 the spontaneously generated voltage which varies with curve A of Fig. 11 and delivers its output current to a circuit having conductance equal to that of the adjoining formation and varying in accordance with the curve shown in Fig. 12. The resulting output of the impulse generator consists of current impulses modulated in magnitude and variably spaced with respect to time in the manner diagrammatically shown in Fig. 13(d) of the drawings. impulse pattern shown in Fig. 13(d) with the curves of Figs. 11 and 12 it becomes apparent that the current impulses developed by the generator 25! are modulated in frequency in accordance with variations of the spontaneous potential of the formation adjacent the drill tool and are simultaneously modulated in amplitude in accordance with variations in the conductivity of the formation.

The electrode 255 is connected to the positive output terminal of the impulse generator 251 and is made of a radioactive material. Accordingly, when current impulses of the character shown in Fig. 13(d) pass from the electrode 255 into the mud stream, corresponding amounts of radioactive tracer are dissolved in the mud. Each current impulse has a magnitude that is substantially proportional to the conductivity of the adjacent earth formation and causes the release of a tracer impulse, i. e., a radioactive substance in a determined relatively small amount which is proportional to the magnitude of the current impulse. These tracer impulses are separated by suitable time intervals and succeed each other at a frequency representing the spontaneous potential.

The radioactive tracer impulses are carried by the mud stream to the top of the drill hole and are subsequently detected by means of the meas- By comparing the uring apparatus shown in Fig. 14 of the drawings. That portion of this apparatus shown within the dashed line enclosure 264 is identical with the apparatus shown in Fig. 3 of the drawings and accordingly, corresponding elements have been designated by the same reference numerals. However, the detector and ionization chamber of the apparatus shown in Fig. 14 are of the integrating type and therefore a voltage is obtained across the resistor 91 which represents the energy of radiation of the detected tracer substance. The radioactive tracer substance arrives in pulses in accordance with the diagram of Fig. 13(d), and each tracer pulse produces a voltage pulse across the resistor 97 which is proportional in magnitude to the amount of radioactive substance in the tracer pulse. There is thus obtained across the output terminals of the resistor 91 a succession of voltage pulses varying in amplitude and frequency in the manner shown in Fig. 13(d). The voltage pulses thus developed across the resistor 91 are impressed upon the input terminals of a frequency measuring network 265 and a rectifier 261 in parallel.

Like the network 98 shown in Fig. 3 of the drawings, the network 265 develops a voltage at its output terminals which is proportional in magnitude to the frequency of the voltage applied to its input terminals. Thus a voltage is produced across the output terminals of the network 265 which varies in magnitude in accordance with variations in the spontaneous potential of the formation being drilled, i. e., in accordance with curve A of Fig. 11. This voltage is metered by a conventional millivoltmeter 266, which may be scaled to indicate the measured spontaneous potential directly. In combination with a resistor 268 and condenser 269, the rectifier 261 functions to convert the pulsing voltage impressed across its input terminals into a continuous direct voltage having a magnitude which varies in accordance with variations in the amplitude of the pulsing input voltage. This continuous direct voltage is developed across the condenser 269 and metered by a millivoltmeter 270. Thus the millivoltmeter 210 is controlled to indicate the conductivity of the formation being drilled.

While different embodiments of the invention have been described, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. Apparatus for determining values of one of the characteristics of formations traversed by a drill hole which extends from the earths surface into the earth, including in combination with a drilling fluid circulating system a rotary drilling tool comprising: a drill collar and a drill bit; means secured near the end of said tool and encased in said drill collar and operative in response to said values for releasing into the fluid traversing said system a tracer substance in an amount representing said values of said characteristic; and measuring means positioned at the earths surface in the vicinity of said circulating system for measuring the amount of tracer substance in said fluid, thereby to provide an indication representing said values of said characteristic.

2. Apparatus for determining one of the characteristics of a formation traversed by a drill hole which extends from the earths surface into s ingzfluidizcirculatingisystem a rot comprisingz i 'aildjtill? collare and electrodes :Iin'sulated i oner sfrom fiesecured near the-flfind ofxsaidr tool i :ielectricat'fcontant withiethei-fluid trai ers tsystem, onset-saidelhtrodes comprising a? s e'eleasing :"subs-tance; Java, rcurrent generator dis- :i posed -=withinsaid adrill rcdllar ivand lhaving its .-outputterminais connected-"tozzsaidr l'e' rodes .t0; .produce an -electrical current flow between said electrodes having ,raf magnitude representing saidl -characteristic; said: current flowing: 'fiom one of; said: electrodes =to'-7the other through a .:portion of the @fluid traversing a sa'id system, I l H W thereby to: release Iromfl'said' one electrode into dioactive 'ions "fforfibro- -the fluid traversing said system: said tracersubducifig ariifidication representative ofsaid value 'stance in an f amount representative of 1 said "of said ""charak'ateris'tic of the geological""foriiia- :characteristic; and? measuring means positioned "tion siirrbunding said adja-c'ntbortion ofsaid -"at the-earths surface iin' the vicinity of "said"cirigo v-culating :1 system :for measuring the amount of tracer substance in said fiu-id; thereby-to provide :anindication representing said characteristic.

:"3. 'Appara-tus for determi-ning one of --the char (1 hole, "comprising: a drilling ;a;cteristics moi. Fa l formation :i-traversed'iby -va drill 25 tool(means for" circulating f drilling fluid in said :11016 which :extends "from the earth's sur'faee hole to elevate cuttings from said drilling r0101 into the earth,-K. i-ncIu-ding in: combination with a -to the "earths' surface; ';a-fieutren*sourecarried :drillin-g fluid circulating s-system a rotary' drillby-said o1 or 'irradiatin I he formation'cutwing-toolncomprising: a dri11-c0l1ar"" and fa dr-ill tings said drilling o 'to ffiroduce'radioacobit; twoelectrodes:insulatednnefrom the other :3 t'ive isotofies in said' cuttings vvhich" are tamed and secured -to said' tool:near-the -end thereof to the surface of the earth by the said fluid travin effective :electrical contact with the fiuid er's'ifig sai'd sy'stmj arid n iaiis afi the'ai'thssurtraversing-said -system, one of *said'electrodes tae hon fi said iadiafive mprisin a r di ive su nce; u'rr fit isotopes for produ ing an rnagcaudngrepiesnm- *gener-a-tor vdisposed*vvithin said -dri1l co11ar "a'Hd 3 ti've ofsai chara'tfiristicfof the geological forhaving its output terminals 1 connected to said *niation surrounding the adj seem portion of said electrodes :to produce an 'electrical current new hole. between said-electrodesPhaving a ma'gnitude rep- '7. Apparatus fordri llirig "a bore hole in the v resenting: said eharacteristic; s'ai-d current 'fiowearth and '-for concurrently I 'dtr'inin ing "one of ing from waneef saidvelectro'des "toithe Tether 2 the characteristics"of-thegeological formation ,throughajportion-of'thezfluid traversmgjsaid suirouridirig said 'ho'le, comprising; --a drilling ssystem, thereby :to 3 release from i said Jone electool; means for circulating" drilling fluid in said 'trode' into :the "fluid traversing asaid syste'm rahole to elevate cuttings from said "drilling ftool .dioactive ions an ::amount "representing said to the ai'tns su'ifa'cejineanscarried by said tool "characteristic; :and measuring means-"positioned andre's onsive{to"c n'gesof said'characte'ristics ratithe arthsz surface in the vicinity of 'said eir- "for iiitemiitt y releasing radioactive-substance :culating systemforfmeasuringtne at'ion of into the*fiui d*tfaversing said syst'em, at afrethe ions in said fluid therebv'to p 'vid'e an inquency representative of said characteristic,'said 5 eleva -eeirt dication;representin Lsaidrvharacteristic. radioactive siil'ost'nce heirig elevated "to the '4. Apparatuslfor iirilling 'a more hole inthe 'earth s-*su1 face by "the drilling fluid traversin earth: and forsimultaneously 'explorifig the g'eosaid "-system; and "means at the 'earths surface logical forma'tions vsurrounding "said "hole, inrisive *to radiation fioin said radioactive :cludingin comb'ination: a drilling tool; i'nans sii s an e anfdto the frequency ofoccurrence of for:circu1atin'g dri11ing flfiid'in said-"hle-tb levatecuttings from sai'd diilling toot to theea'i ths V a "o'duicingflan indicationrep- SurfaCe; an electrolytically 'diss'oluble eldtfdde l'snbitive f s'aid 'h acteristic.

"carried m and insulated from said m n a'n'd *-8.-Kfi"15afa"t s 'fo'r s V adapted 1:0 make l'e'ctrical contact with Said 'alhth a lid fo'r tidiififffitly df rminihg twbfdifsum; a "current generator carried hy said tool ferent characteristics of the --geolo'gical formaand having its ou-tput terminals respetively g tioh 'surroufidiriesaidigole,-c riipr s n :adrium connected" 'to said tool 'and' said 'eletrode ei-'eto'o'h ea rcirfculatinig drilliiijg fiuidinsaid by the curren t "output or 'said 'ge'nerator flows libletoelevate'euttings ffofnsaiddrilling*tool to :irom saidlctrode irito*said fluid tO eItitfDI-fii the eaftfis surface; frieiiistietfiid by said tool call-y 'diS'SOIVe material fm'm said lefifdiie in and 'al'tiiifi'd in rt$p5n$ "170 said characteristics said fiuid; m'eans to var thburrrit oii-tbiit of forir'iti-"inittehtlv rele sing radioactive substance said generatorin accordance with a cha'ngiiig int0"the-'fl1iid' t vrsiirg said system "at'a frecharacteristic *of said forma'tion, whereby the qseneya-e resemauve"arone oreamc aracterisamount of said electrode material electrhlytitics ahd inhu''aiii titii'a s' epifes'eh'tative'of the'other cally diss'olv'ed in said' fi'uid is varied-edffesporidof said chai 'aeti'is'tis clradioactive-substance ingly; and me'an s atthe earth ssurfaceidr tesv '70 bi'ri'ge ii to' t eearths'su'rfaceby the drilling said circulating fiu'id to "determine the fig fl-iiid traversing "said yst'em; nie'ansat the amount "of said di'ssolved material 'oontained in earth's surface responsive to the frequencv 'f'd said fluid and thus "obtain "a measuredunuica- "adioac vesubstancein the fluid tion of-saidcharacteristic. r prouuemgan indicai5. Apparatus fo'r drilling a b'ore hole i'n the tion ibfesenteitive of said the 6f "said ch'arac teristics; and means at the earths surface responsive to the quantities of radioactive substance in the fluid traversing said system for producing a second indication representative of the said other of said characteristics.

9. Apparatus for drilling a bore hole in the earth and for concurrently determining one of the characteristics of the geological formation surrounding said hole, comprising: a drilling tool; means for circulating drilling fluid in said hole to elevate cuttings from said drilling tool to the earths surface; means carried by said tool and actuated in response to said characteristics for releasing a radioactive substance into the fluid traversing said system, having a measurable characteristic representative of said characteristic of the geological formation surrounding the adjacent portion of said bore hole, said substance being elevated to the earths surface by the drilling fluid traversing said system and having a half life exceeding the time required for the substance to reach the earths surface and less than the time required for any incremental portion of the drilling fluid completely to traverse said system; and means at the earths surface responsive to said measurable characteristic of the radioactive substance elevated to the earths surface for producing an indication representative of said characteristic of the geological formation surrounding said adjacent portion of said hole.

10. Bore hole drilling apparatus including: a drill collar; a pair of spaced-apart electrodes mounted upon said drill collar, at least one of said electrodes being insulated from said drill collar and at least one of said electrodes comprising an electrolytically dissoluble material containing a radioactive tracer material.

11. Bore hole logging apparatus for logging a bore hole in conjunction with a carrier fluid circulating through the hole, including: a pair of spaced-apart electrodes, at least one of which comprises an electrolytically dissoluble material capable of releasing a detectable tracer into said carrier fluid. upon the application of a voltage across the electrodes; and voltage supply means for supplying a voltage difference across said electrodes.

12. Bore hole logging apparatus for logging a bore hole in conjunction with a carrier fluid circulating through the hole, including: two pairs of spaced-apart electrodes, at least one electrode of each pair comprising electrolytically dissoluble material capable of releasing a detectable tracer upon the application of voltages across the said electrode pairs while in contact with an electrolytic fluid; and separate voltage supplying means for supplying voltage differences across each one of said pairs of electrodes.

13. Bore hole logging apparatus for logging a bore hole in conjunction with a carrier fluid circulating through the hole, including: a, pair of spaced-apart electrodes adapted to be lowered into a bore hole and into electrical contact with said fluid therein, at least one of said electrodes comprising a material containing a detectable radioactive tracer and electrolytically dissoluble into said fluid upon the application of a voltage across the said pair of electrodes while in contact with said fluid; and voltage supply means for supplying a voltage difierence across said pair of electrodes.

14. Apparatus for determining the value of a variable physical quantity within a borehole out of which a stream of fluid is flowing, comprising:

means located within said borehole and operative in response to said value for releasing into the said stream of fluid within said borehole a tracer substance in an amount varying in a manner representative of said value of said physical quantity; and means positioned at the earths surface in the vicinity of said stream of fluid flowing out of said borehole responsive to the said varying amount of tracer substance arriving at the earths surface in said stream of fluid, to provide an indication representative of said value of said physical quantity.

15. Apparatus in accordance with claim 14 in which said tracer substance is radioactive.

16. Bore hole logging apparatus including: a supporting structure adapted to be lowered into a bore hole; a pair of spaced-apart electrodes mounted upon said structure, at least one of said electrodes being insulated from said structure and at least one of said electrodes comprising an electrolytically dissoluble material containing a detectable tracer material.

1'7. Bore hole drilling apparatus including: a drill collar; two pairs of spaced-apart electrodes mounted upon said drill collar, said electrodes of each pair being insulated from each other, at least one electrode of each said pairs of electrodes being insulated from said drill collar, and at least one electrode of each said pair of electrodes comprising an electrolytically dissoluble material containing a detectable tracer material.

18. Borehole logging apparatus comprising: means for circulating a carrier liquid through the borehole from a subsurface zone to the top of the borehole; means is said subsurface zone for passing an electric current through the carrier liquid in said zone, which current varies in accordance with the value of a borehole quantity to be logged, said current passing means including an electrode in contact with the carrier liquid which electrode is electrolytically dissoluble in said liquid and capable of forming a radioactive tracer substance upon being irradiated by neutrons; a source of neutrons adjacent said electrode; and means responsive to varying quantities of said tracer substance in the liquid for obtaining an indication which is characteristic of the value of said borehole quantity.

l9. Borehole logging apparatus for logging a borehole in conjunction with a carrier fluid circulating through the borehole, including: a pair of spaced-apart electrodes adapted to make contact with said carrier fluid, at least one of which electrodes is electrolytically dissoluble in said carrier fluid and. comprises material capable of forming radioactive isotopes upon being irradiated by neutrons, and thus is capable of releasing said radioactive isotopes into said carrier fluid upon application of a current across said elec trodes; a neutron source in said apparatus adjacent said one of said electrodes; supply means for supplying a current across said electrodes, which current varies in accordance with the value of a borehole quantity to be logged; and means responsive to varying quantities of said radioactive isotopes thus released into the fluid for obtaining an indication which is characteristic of the value of said borehole quantity.

20. Apparatus for determining different characteristics of formations surrounding a bore hole including: supporting structure including a drill bit and drill collar insulated from each other and adapted to be lowered into contact with fluid in a bore hole; a first pair of spaced-apart electrodes mounted upon and insulated from said structure 25 and positioned so that a. current flowing from one electrode to the other through said fluid will traverse the adjacent formation, at least one of said electrodes being capable of electrolytically releasing into said fluid a detectable tracer material having certain characteristics, upon the application of a voltage across the said pair of electrodes; a second pair of spacedapart electrodes insulated from each other and from said first pair of electrodes, at least one of said second pair of electrodes being capable of releasing electrolytically into said fluid, upon application of a voltage across said second pair of electrodes, a detectable tracer having characteristics different from that released from said one of said first pair of electrodes; means for supplying a voltage across said first pair of electrodes; means for supplying a voltage across said second pair of electrodes; and means electrically connected between the drill bit and collar and actuated by the voltage difference appearing therebetween for controlling and varying said last mentioned voltage supplying means in accordance with said voltage difference appearing between said drill bit and collar.

JAN JACOB ARPS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,720,325 Hackstaff et a1 July 9, 1929 2,225,668 Subkow et a1. Dec. 24, 1940 2,231,577 Hare Feb. 11, 1941 2,263,108 Stuart Nov. 18, 1941 2,335,409 Hare Nov. 30, 1943 2,337,269 Piety Dec. 21, 1943 2,339,129 Albertson Jan. 11, 1944 2,341,745 Silverman et al. Feb. 15, 1944 2,342,273 Hayward Feb. 22, 1944 2,354,887 Silverman et a1. Aug. 1, 1944 2,374,197 Hare Apr. 24, 1945 2,468,905 Warren, Jr May 3, 1949 

